1. Field of the Invention
The present invention relates to fabrication of semiconductor devices, and most specifically to the fabrication of semiconductor devices at sub-lithographic sizes.
2. Description of Related Art
The operation of Moore's Law, which holds that the number of transistors on an integrated circuit will double every 18 months, clearly requires ever smaller devices. As is well known, current fabrication processes are encountering limitations in terms of the smallest devices possible using known techniques. Conventional processes are built around deposition and etching, employing photoresistive etch masks. Generally, layers of materials are deposited and then covered with a photoresistive material. A pattern is projected on the photoresist using visible light, which alters the structure of the photoresist so that unwanted material can be easily removed, and the resulting pattern can be etched into the underlying material.
The smallest feature that can be formed using a given process is referred to as the “minimum feature size.” As size requirements have become smaller, however, the need for smaller feature sizes has come up against problems such as the wavelength of light—one cannot resolve objects smaller than the wavelength of the light being used. Wavelengths have been reduced, now extending below the visible spectrum, and substitutes for conventional projection have been adopted.
The art has recognized this problem but has not presented a solution that allows formation of features in the range of 100 nm and less. For example, U.S. Pat. No. 6,744,088, to Dennison, entitled “Phase Change Memory Device on a Planar Composite Layer” discusses the minimum feature size issue and presents a number of possible solutions, including using shorter-wavelength sources for the lithography, such as x-rays, or phase shift masks, or sidewall spacers, all of which suffice down to approximately 100 nm. No solutions below that level are offered, however.
Problems have arisen in manufacturing such devices with very small dimensions, and with variations in process that meets tight specifications needed for large-scale memory devices. It is desirable therefore to provide a memory cell structure having small dimensions and low reset currents, and a method for manufacturing such structure that meets tight process variation specifications needed for large-scale memory devices. It is further desirable to provide a manufacturing process and a structure, which are compatible with manufacturing of peripheral circuits on the same integrated circuit.